The present invention relates to image drawing apparatus and method thereof whereby an anti-aliasing process is executed. The anti-aliasing process is a process for removing undesirable jaggedness or "stair-step" (shaped like the steps of a stair) appearing on the edge of the output image. The present invention particularly relates to an image drawing apparatus and method by which a high speed anti-aliasing process can be realized. One type of the image drawing apparatus is a vector image drawing apparatus. Hereinafter, "vector" means a line (it can be a straight line or a curved line) having a certain length and position and extending in a certain direction, and "a vector image" means a image comprising such a vector or vectors. A vector comprising a straight line is referred to as "a straight line vector" and a vector comprising a curved line is referred to as "a curved line vector", hereinafter.
Recently, use of a vector image printing system in variable fields has begun, which vector image is treated in a computer graphics process. The reason for this is that the so-called DTP (desk top publishing, meaning a publishing system utilizing the personal computer) has come into wide use. A typical example of the vector image printing system is the system using the "postscript" (a trade name) of Adobe System Incorporated(U.S.A.). The "postscript" belongs to the language group referred to as the page description language (this is referred to as PDL hereinafter), which is the programming language for describing the form of contents constituting a document page. The form of contents includes the text (a part comprising characters) and the graphics, both of which are included in the document page, and the form of contents also includes the arrangement and style of the text and graphics. A vector font is applied as the character font for such a system as mentioned above.
Applying such a vector font as the character font improves the character printing quality (even if the dimensions of the character are enlarged or reduced to various sizes) much more than the system in which the bit-map font (for example, the conventional ward processor) is applied as the character font. Further, applying such a vector font enables a printing in which the character font and graphics are combined together.
However, most laser printers or other printing apparatus used for such a vector image printing system have resolutions of not finer than from 240 dpi (dot per inch) to 400 dpi. Such a coarse resolution may cause undesirable jaggedness or "stair-step" to appear on the edge of the printed figure. Such a phenomena is referred to as "aliasing". There is a method whereby an anti-aliasing process is executed to obtain a more beautiful (having a smooth edge) character printing image. In such a process, a darkness modulation is performed for the part where such a stair-step (jaggedness) appears on the edge of the printed figure. Thus a character printing image which appears smooth to the eye can be obtained. "Darkness modulation" means that the darkness of each pixel is controlled in accordance with a below mentioned area ratio.
The averaging filtering method, the weighted filtering method, the convolution method, or some other method is generally applied to execute the anti-aliasing process for the vector image drawing apparatus in art related to the present invention.
The averaging filtering method is described below. Each pixel is divided into sub-pixels such as N*M (N and M are, respectively, natural numbers), that is, each pixel has N rows and M columns of sub-pixels. Then, after a raster calculation so as to keeping the fine resolution is executed, the darkness of each pixel is obtained as a result of averaging the darknesses of the respective N.noteq.M sub-pixels.
An averaging filtering method whereby the anti-aliasing process is executed, is described more concretely with reference to FIGS. 1A and 1B. In a case where the edge of the image overlaps the pixel (for example in FIG. 1A, where the image overlaps a pixel P.sub.1, the image presents in the right bottom side of the line L.sub.1, that is, the right bottom side of the line L.sub.1 is to be painted to be dark), if the anti-aliasing process is not executed, then the maximum darkness of indicatable tones (for example, k=255, if the maximum number of tones is 256) is allocated for the darkness k of the pixel P.sub.1, as shown in FIG. 1A.
In the averaging filtering method in which N=M=7 whereby the anti-aliasing process is executed for the pixel P.sub.1, the following method is executed: the pixel P.sub.1 is divided into 7.noteq.7 sub-pixels; then the sub-pixels (hatched) overlapped by the image are counted; the number of the counted sub-pixels (this number is 28 in the example of FIG. 1B) is then divided by the number of all the sub-pixels in the pixel P.sub.1, the number of all sub-pixels (this number is 49 in this example), that is, the darknesses of the 49 sub-pixels: in the pixel P.sub.1, then being standardized (averaged). The result of this division, that is the area ratio (approximately 0.57, in this example), is then multiplied by the maximum darkness (255), so that approximately 145 is then obtained, that is, the calculation of the darkness k (145) of the pixel P.sub.1.
As mentioned above, the darkness k of each pixel is determined in accordance with the number of the sub-pixels in each pixel, which sub-pixels the image overlaps.
A problem, in applying the averaging filtering method of the vector drawing apparatus to execute the above mentioned anti-aliasing process, is described below. A long time is required to calculate the area ratio, which area ratio is calculated by the following method: the pixel is divided into a plurality of sub-pixels (for example, 49 sub-pixels); the sub-pixels overlapped by an image are counted; and then the area ratio is obtained. This results in it being difficult to improve the indicating speed and/or the printing speed. Particularly in the above mentioned convolution method, it is difficult to improve the processing speed because a long time is needed to calculate the area ratio and also because a process for each pixel includes a process with regard to another plurality of pixels.